Maximum current when charging a device through a computer USB 3 port using a USB 2 cable?

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Just got a new MacBook with USB 3 ports. I thought it would be able to charge my Nexus 4 phone much faster using USB 3 900 mA current.

However, I haven't seen any improvements compared to charging on a USB 2 port. Obviously, AC chargers can charge way over 500 mA using USB 2 cables.

Which brings me to question: can a (micro) USB 2 cable hooked up to a computer USB 3 port charge a device at 900 mA current, or is it limited by the USB 2 cable to 500 mA?

djule5

Posted 2014-12-30T17:36:19.210

Reputation: 109

Question was closed 2015-01-07T10:03:58.593

@Ƭᴇcʜιᴇ007 Right, the question is really similar but I don't feel like it has been answered correctly. I've read all the comments but there isn't a clear answer. I feel like there should be some Internet source/reference/spec that precisely answers my question. I would rather not have to use a current multimeter to find it out. – djule5 – 2014-12-30T17:57:31.287

Just because the current answer(s) don't satisfy you doesn't make it a different question. If you'd like newer/different answer to the existing question, please consider placing a bounty on it.

– Ƭᴇcʜιᴇ007 – 2014-12-30T18:03:47.527

@Ƭᴇcʜιᴇ007 Fair enough, but I do think my question is asking for something more precise than the other one i.e. specifically asking for the maximum current possible. – djule5 – 2014-12-30T18:20:29.100

a Mac can output over 2 amps, much more than USB spec (even over USB 2), but the device has to know how to ask for it. [I very much doubt Apple will put much effort into making sure a droid matches the spec] – Tetsujin – 2014-12-30T19:22:08.190

@Tetsujin Okay, that's weird though because even Apple says charging iPads through a computer is slower and recommend using the wall charger which provides 2.1+ A. – djule5 – 2014-12-30T20:51:20.337

hmm… it may only be 1100mA on USB 2 - 2.9A on USB 3. This isn't particularly clear http://support.apple.com/en-us/HT4049 but I am sure that the device must be able to request the power.

– Tetsujin – 2014-12-30T21:02:23.180

@Tetsujin You're right, although it seems to be 1100 mA regardless of USB 2/3 for some Apple devices (e.g. iPad) connected directly into Apple computers (not hubs). – djule5 – 2014-12-30T23:20:23.387

Answers

2

Per Apple: http://support.apple.com/en-us/HT201163#12

USB 2 cable is usually limited to 500 mA. It's about how many wires are in the cable, and the Mac limiting power accordingly (power management). A wall charger generally lacks logic, so it's full power all the time.

I checked with a variable load, and it seems to hold true (480 mA typically)

user403201

Posted 2014-12-30T17:36:19.210

Reputation: 21

I came across that page too and can't find a more reputable source. A few other sites seems to confirm this as well. So from what I understand (correct me if I'm wrong), the 900 mA current is only available by using both a USB 3 port AND cable. These power restrictions apply to reduce interference when data transfer is required. AC/wall chargers don't deal with data and thus can deliver much more current, regardless of the USB cable version. See https://en.wikipedia.org/wiki/USB#Charging_ports for more info.

– djule5 – 2014-12-31T00:58:47.663

Asked: 2014-12-30T17:36:19.210

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Active: 2014-12-30T21:05:09.913